5E3 - OK using smaller potentiometers? (rated for less power)

[waspclothes]

Hey guys,
Is it OK to use smaller pots rated at only .1W in a 5E3?

[eschertron]

Look at the AC and DC voltages that are likely to be present across those components. That'll give you one answer. Current capability may not be the only criterion for component choice, though.
Some manufacturers I (when feeling spendy) prefer, not just because the pots are rated higher, but because the pots look to be hermetically sealed and will last 50 - 60 years or more.

[waspclothes]

Look at the AC and DC voltages that are likely to be present across those components. That'll give you one answer. Current capability may not be the only criterion for component choice, though.
Some manufacturers I (when feeling spendy) prefer, not just because the pots are rated higher, but because the pots look to be hermetically sealed and will last 50 - 60 years or more.

Thanks for the reply. I'm just planning a 5E3 build at this stage, so I can't measure any voltages where the pots are. I agree though, the more expensive sealed pots can be very nice!

[Alan0354]

From the schematic of 5E3, all three are 1M pot that does not carry any DC voltage, just AC signal. On top the signal amplitude only go through 1 stage of amplification, it's not very high. The gain of the first stage is about 50, if the guitar output is 0.5Vpp, output at the plate of the tube is about 25Vpp. I would be comfortable to use the 16mm pot instead of the big 24mm. I don't know what is the wattage rating.

[Leo_Gnardo]

Hey guys,
Is it OK to use smaller pots rated at only .1W in a 5E3?

In general, controls in a guitar (or hi fi) amp handle tiny fractions of a watt, so your 1/10 W rated pots aren't going to go up in smoke, don't worry about that. As Eschertron mentioned, quality level may be better with higher-wattage rated parts, considering accuracy of curve (linear or audio) and lifetime (# of turns before track wears out.)

FWIW Mesa has been building their amps with crappy 16 mm pots for quite a while now, over 20 years. As far as I've seen, they work just fine, unless they get smashed. Half the time I can reassemble a bashed-in one but when I replace, it's a 24mm pot that goes in.

[Alan0354]

I always question about the power rating. My guess is the low wattage rating of potentiometers are not because the element cannot take the power. It's more if you try to draw current from the wiper that it cannot handle the current because the contact area is very small and you create a hot spot. Just looking at the size of the element in a 16mm pot, it's bigger than a 1W resistor. The element should be able to withstand more power and voltage.

My guess is the pots can handle more power across both ends, limitation is only how much current you can take from the wiper......Particularly if the wiper is position close to one end. Then the part of the element between the wiper and the end is very small and cannot handle much power.

[Chuck H]

Well... They (the proverbial, in this case pot manufacturers) do make .25W and .5W, good quality 16mm pots. The OP never mentioned 16mm as a size anyway. "They" also make lower quality 24mm pots rated at .1W. Soooo...

What is the reason for the .1W rating criteria? Are you trying to save money? Are these parts you have on hand or are more readily available to you? Does it happen to be the rating of the parts you looked at already (there are other choices)?

It's not hard to get good pots. When weighed against the effort of building an amp it's a no brainer to spend the extra five or ten bones to be sure you're not going to open any lug connections with an improper iron temp or multiple solderings while troubleshooting or modifying.

JM2C

[Alan0354]

I don't even think there is much difference in price between 16mm and 24mm. Forget CTS, I use all Alphas, I use 16mm. They are cheap enough. It's not worth getting the no name Chinese brand......and this is speaking from one that is born cheap.

[Chuck H]

Alphas have been hit or miss for me. I use them often enough anyway. But they are about as low in quality as should be tolerated. I've ruined a few just building an amp when a lug may have multiple components and leads. The extra solder required and added heat sinking of the extra metal can make for a longer heat exposure to where the lug connects to the resistance track and they fail. Now, a few pots out of what is probably a couple of hundred isn't too bad. But I've ruined several Alpha pots with multiple soldering. It's common enough that I don't even start a project that might ruin a pot unless I have another, new one, on hand.

Just like food for your face or gas for your car. Quality will make everything better. You want good components in your amp where it makes the most sense. IMO there aren't many places to cut corners for an amp build. I value my time too much. Why even build your own amp if quality isn't part of the goal? I can buy cheaply built amps all day. And they sound pretty good too.

[Alan0354]

I try not to experiment with my final build. I always build the prototype into my Bassman to experiment. I never have a failure with Alpha yet. I do have quite a few failure with the old pot even from Fender SF. I don't even think heat is the problem, it's the bending of the lug that stress the element and crack it. At least I think that's what happen to me. So I keep a draw of old pots taken out from old amps and guitar, I put them in the Bassman and work it over. Then I use brand new Alpha pots for new build and expect to experiment very little with it.

[Alan0354]

Speaking of pots, I am looking for a dual gang 1M potentiometer that can withstand 500V across the two ends. I don't need to draw current from the wiper, just voltage. Any suggestion?

[Chuck H]

Why not stay with your usual brand?

RV24BF-10-15R1-B1M Alpha (Taiwan) | Mouser

[Alan0354]

Why not stay with your usual brand?

RV24BF-10-15R1-B1M Alpha (Taiwan) | Mouser

Thanks you, I have been looking and looking. You always have the answer for me.